What are the characteristics of obligate parasites?

Obligate parasites are organisms that have evolved to depend entirely on a host for their survival. This characteristic means that they cannot complete their life cycle without exploiting a host organism. They integrate into the host’s biological systems to extract nutrients and reproduce.

This reliance on a host distinguishes obligate parasites from other types of parasites or organisms. For example, some organisms might be free-living and capable of thriving independently (as seen in the first option), while obligate parasites cannot establish themselves outside a host. Additionally, unlike some organisms that can produce their own nutrients (as the third option suggests), obligate parasites lack the necessary functions to survive without the resources provided by their hosts. Finally, while some organisms can adapt to extreme environments, this adaptability does not reflect the defining characteristic of obligate parasites, which is their absolute necessity for a host to sustain life.

Therefore, the defining feature of obligate parasites is their exclusive reliance on host organisms for survival and reproduction.